The present invention relates to a technical field of a pretensioner (hereinafter, sometimes referred to as xe2x80x9cPTxe2x80x9d) which rotates a reel shaft of a seat belt retractor in a belt-winding direction by reaction gas at an early stage in emergency event such as a vehicle collision, thereby increasing the force of restraining an occupant by a seat belt and, more particularly, relates to a technical field of a pretensioner comprising a piston which receives gas pressure of reaction gas and thereby produces force, and a plurality of force transmitting members for transmitting the force produced by the piston to a reel shaft of a seat belt retractor.
Vehicles such as automobiles are equipped with seat belt devices. In recent years, various kinds of seat belt devices with pretensioners have been developed. A pretensioner is a device for rapidly removing slack of a seat belt and also applying tension to the seat belt to increase the force of restraining an occupant by rotating a reel shaft of a seat belt retractor in the belt winding direction by the reaction gas generated by a gas generator at an early stage in emergency event such as a vehicle collision.
As an example of conventional pretensioners, a pretensioner has been proposed in Japanese Patent Unexamined Publication No. 2001-63519 (incorporated by reference herein), which comprises a piston which receives gas pressure of reaction gas and thereby produces force, and a plurality of force transmitting members, composed of balls, for transmitting the force produced by the piston to a reel shaft of a seat belt retractor.
FIGS. 4(a) and 4(b) disclose a pretensioner disclosed in the aforementioned publication, wherein FIG. 4(a) is a sectional view showing the pretensioner in non-operational state and FIG. 4(b) is a sectional view similar to FIG. 4(a) but showing the pretensioner during winding-up a seat belt.
FIGS. 4(a) and 4(b) disclose a pretensioner (PT) 1, a pipe 2 having an open end 2a, a plurality of force transmitting members 3 which are movably disposed to be in contact with each other within the pipe 2 and are each composed of a metal ball made of iron, aluminum, or the like. A piston 4 is movably disposed within the pipe 2 to receive gas pressure of reaction gas and is composed of a ball made of material having sealing function such as silicone rubber and resin. A pressure vessel 5 having a pipe-like shape is connected to a proximal end 2b of the pipe 2. A gas generator 6 is housed in the pressure vessel 5. A ring gear 7 is positioned to enter into the pipe 2 through a cut-out portion 2c formed near the end of the pipe 2. Levers 8, 9 project from the outer periphery of the ring gear 7. Internal teeth 10 are formed on the inner peripheral surface of the ring gear 7. FIGS. 4(a) and 4(b) also disclose a reel shaft 11 of a seat belt retractor and a pinion 12 fixed to the outer periphery of the reel shaft 11. External teeth 13 are formed on the outer periphery of the pinion 12. A casing 14 is provided for receiving the force transmitting members 3 out of the pipe 2. A gear holder 15 is provided for retaining the ring gear 7 in a home position where the internal teeth 10 of the ring gear 7 do not mesh with the external teeth 13 of the pinion 12. A pair of pins 16, 17 are studded on the gear holder 15 for holding levers 19, 20 therebetween.
The pipe 2 forms a tube path curved in substantially flat oval shape. The force transmitting members 3 and the piston 4 housed in the pipe are guided to move along the tube path. Generally, the PT 1 is integrally installed in a frame of the seat belt retractor.
In the conventional PT 1, the front-most force transmitting member 3 is held in a position where it is in contact with the lever 8 of the ring gear 7 as shown in FIG. 4(a). The other force transmitting members 3 and the piston 4 are held in the illustrated state when the PT 1 is not operating (normal state). In addition, the levers 9 of the ring gear 7 are held between pairs of pins 16, 17 of the gear holder 15, respectively, in order to retain the ring gear 7 in the home position, shown in FIG. 4(a). As a result, the internal teeth 10 of the ring gear 7 are held in a position in which they do not mesh with the external teeth 13 of the pinion 12.
As the vehicle suffers an emergency event such as a collision, the event is sensed and the gas generator 6 is actuated to generate reaction gas into the pressure vessel 5. The gas is introduced into the pipe 2 and directs toward the piston 4 as shown by an arrow A. The piston 4 receives the gas pressure of the gas so as to produce force pushing the force transmitting members 3. The force is transmitted to the lever 8 of the ring gear 7 via the force transmitting members 3 which are in contact with each other.
The piston 4 is deformed easier than the force transmitting members 3 because the force transmitting members 3 are made of metal for achieving effective force transmission and the piston 4 is made of material such as resin for enabling the enlargement of the diameter of the piston 4 to seal the tube and prevent gas leak by. Therefore, during the force transmission, the piston 4 is deformed in the operational direction of gas pressure by the gas pressure and the reaction force from the force transmitting member 3 adjacent to the piston 4. The diameter of the piston 4 is enlarged so that the pressure at the contact area of the piston 4 relative to the inner peripheral surface of the pipe 2 is increased, thereby creating the sealing function for sealing the gas pressure.
As the pushing force is transmitted to the lever 8 of the ring gear 7, torque in the belt-winding direction B (the counter-clockwise direction in FIGS. 4(a), 4(b)) is produced on the ring gear 7. The pins 16, 17 holding the levers 19, 20 are sheared by the torque so as to release the ring gear 7 from being retained in the home position by the gear holder 15. Therefore, the ring gear 7 moves to substantially the right in the drawing, whereby the internal teeth 10 of the ring gear 7 are meshed with the external teeth 13 of the pinion 12 as shown in FIG. 4(b).
The ring gear 7 rotates in the belt-winding direction (counter-clockwise direction) because the lever 8 is pushed through the force transmitting members 3 by the pushing force of the piston 4 receiving the gas pressure. Because of the rotation of the ring gear 7, the pinion 12 starts to rotate in the same direction so that the reel shaft 13 starts to rotate in the same direction, thereby starting the winding-up operation of the seat belt.
The respective force transmitting members 3 and the piston 4 move in the counter-clockwise direction according to the rotation of the ring gear 7 due to the pushing force of the piston 4 receiving the gas pressure. The front-most force transmitting member 3 is positioned between the lever 8 and an adjacent lever 9 and the next two force transmitting members 3 are positioned between the lever 9 adjacent to the lever 8 and the next lever 9. After that, subsequent two force transmitting members 3 fit in the space between subsequent two levers in the same manner. The force transmitting member 3 out of the engagement with the levers 8, 9 is forced out from the end 2a of the pipe 2 and is received by the casing 14.
Due to rotation of the reel shaft 13, the seat belt is wound up, thereby removing slack of the seat belt and also applying tension to the seat belt to increase the force of restraining the occupant.
In the conventional seat belt retractor having such a pretensioner, the seat belt is wound up in a predetermined manner by the operation of the aforementioned pretensioner and subsequently the seat belt is also loaded to be withdrawn by inertia force of the occupant so that torque is applied to the reel shaft in the belt-unwinding direction. However, even though the torque is applied to the reel shaft in the belt-unwinding direction, the locking mechanism of the seat belt retractor locks the reel shaft from rotating in the belt-unwinding direction, thereby preventing the seat belt from being withdrawn. A variety of seat belt retractors have been developed, which are provided with an energy absorbing (hereinafter, sometimes referred to as xe2x80x9cEAxe2x80x9d) mechanism for absorbing impact energy in order to soften the impact which is applied against the occupant by the seat belt stopped from being withdrawn.
Generally, EA mechanisms to be integrally assembled in the retractors include a torsion bar. The torsion bar is designed such that one end thereof is locked from rotating in the belt-unwinding direction by the locking mechanism when actuated and the other end thereof is rotated together with the reel shaft in the belt-unwinding direction so that the torsion bar is twisted and deformed to allow the seat belt to be withdrawn, thereby absorbing impact energy.
The restraining force on the occupant and impact absorption can be improved by adapting the PT 1 as disclosed in the aforementioned publication to the seat belt retractor with such an EA mechanism.
However, certain problems result from adapting the PT 1 to a seat belt retractor having an EA mechanism. The operation of a seat belt retractor having both a PT 1 and an EA mechanism will now be considered. As shown in FIG. 5(a), the piston 4 receives gas pressure from the gas generator 6 during the operation of the PT 1, whereby the piston 4 is pressed against the adjacent force transmitting member 3. At this point, since the contact face of the force transmitting member 3 relative to the piston 4 is spherical and the force transmitting member 3 is harder to be deformed than the piston 4, the piston 4 is expected to be deformed in the operational direction of gas pressure (the force transmitting member 3 side of the piston 4 is collapsed and deformed) as shown in FIG. 5(b). Then, the deformed portion of the piston 4 may wedge into space between the outer spherical surface of the force transmitting member 3 and the inner peripheral surface of the pipe 2.
As the EA mechanism is actuated soon after the actuation of the PT 1, the reel shaft 11 rotates in the belt-unwinding direction because of the torsional deformation of the torsion bar. Therefore, force produced by the withdrawing of the seat belt is applied to the respective force transmitting members 3 in the direction opposite to that of the operation of the PT 1, as shown in FIG. 5(c). Then, the respective force transmitting members 3 are returned to the original positions so that the spherical surface of the force transmitting member 3 presses the piston 4. Since the piston 4 is easier to be deformed than the force transmitting members 3, the piston 4 is further deformed and the deformed portion of the piston 4 further wedges into the space between the spherical surface of the force transmitting member 3 and the inner peripheral surface of the pipe 2, thus disturbing the smooth movement of the force transmitting members 3.
Though, even when the smooth movement of the force transmitting members 3 is disturbed, the EA mechanism exhibits its EA function itself, it is preferable that the EA mechanism exhibits more effectively its EA function.
The present invention was made for under the aforementioned circumstances and the object of the present invention is to provide a pretensioner having a plurality of force transmitting members, in which a piston as force receiving member is designed to be hardly deformed, thereby allowing further smooth movement of the force transmitting members.
According to the present invention, a pretensioner is provided comprising: a plurality of force transmitting members which are guided along a predetermined tube path and are arranged to transmit force to a reel shaft for rotating the reel shaft in a belt-winding direction; a gas generator for generating gas in the event of emergency; and a piston which is guided along the tube path and receives gas pressure of the gas to push the force transmitting members, wherein the force produced by the piston when receiving gas from the gas generator in the event of emergency is transmitted to the reel shaft via the force transmitting members so as to rotate the reel shaft in the belt-winding direction to wind up a seat belt, and wherein one of the force transmitting members which is directly adjacent to the piston has a flat face to be in contact with the piston.
According to another embodiment of the present invention, all of the other the force transmitting members except the force transmitting member directly adjacent to the piston are formed in balls made of metal such as iron or aluminum.
According to another embodiment of the present invention, the force transmitting member directly adjacent to the piston is formed into any one of a barrel-like shape, a (column+hemisphere) shape, and a (truncated circular cone+partial sphere) shape.
Further, according to yet another embodiment of the present invention, the force transmitting member directly adjacent to the piston is made of metal such as iron or aluminum.
Furthermore, according to another embodiment of the present invention, the piston is formed into any one of a ball-shape, a barrel-like shape, a (column+hemisphere) shape, and a (truncated circular cone+partial sphere) shape from resin such as silicone rubber.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only, and are not restrictive of the invention as claimed.